How UE identify various radio links and usage of frame offset and chip offset…

The RNC maintains CFN(Connection Frame Number) for the timing relation between UE and UTRAN .A similar concept is maintained known as SFN (System Frame Number) at the cell level.These two parameters are necessary for the overall synchronization process in the UTRAN.

The SFN has a range from 0 to 4095, whereas the CFN has a range from 0 to 255, i.e. 16 cycles of CFN is equal to 1 cycle of SFN , so modulo 256 arithmetic is applied to derived the CFN from SFN.

Now the question is how these two are inter related ?

Actually  the CFN is used for L2 synchronization between UE and UTRAN , but it is not transmitted to the air interface, both , UE and Node-B derive it from the SFN.

Now the next question is arrived , that how UE will know about the SFN value and what will happen when UE have more than 1 RL, how UE will find the timing between these different RLS, the answer of this , lies in the concept of frame offset and chip offset.Lets first check about the SFN value then we will see how these two offsets are used.

UE gets the value of SFN with the help of System information Blocks, which are transmitted in a cell via BCH channel or say PCCPCH channel.A UE has to read System information Block when it enters into a cell, and from this UE gets the information about the various parameters of the cells including the SFN value, which it finds in the header portion of the message.Once the UE gets the SFN value , it can derived the CFN value withe the help of the formula, CFN =  (SFN  – (DOFF div 38400)) mod 256

Where DOFF is the default DPCH offset , specified in the RRC message(RRC connection setup, in case of soft handover , RL specific frame offset is sent instead of default one…), while the Node-B derives this value, with the help of frame offset and chip offset, so in case of Node-B this DOFF is nothing but the frame offset + chip offset.

Now lets explore the Frame offset and Chip offset funda, the frame offset and chip offset values are nothing but the timing relation between downlink DPCH relative to P-CCPCH.

The frame offset and chip offset value is given to the Node-B during the RL setup(or rl addition) procedure and the same values are also sent to UE in the RRC message(rrc connection setup or through Active set update message). So that both UE and the Node-B know from where to start listening the new RL.

In case of UE , the radio link specific DPCH frame offset becomes more important when UE involves in the soft hand overs, this frame offset will tell the UE ,the new radio frame timing relative to the P-CCPCH  radio frame timing of the new cell.So now UE knows from where to start listening the new radio link frames in the new cell.See the diagram below,